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File-System Structure

File-System Structure. File structure user view Logical storage unit Collection of related information File system physical view Resides on secondary storage (disks) Partitions and mounting Blocks. Moving-Head Disk Mechanism. Low Level Format.

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File-System Structure

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  1. File-System Structure • File structure user view • Logical storage unit • Collection of related information • File system physical view • Resides on secondary storage (disks) • Partitions and mounting • Blocks Operating System Concepts

  2. Moving-Head Disk Mechanism Operating System Concepts

  3. Low Level Format • Low-level formatting, or physical formatting — Dividing a disk into sectors that the disk controller can read and write. • Zones of cylinders allow more (up to 40%) sectors on outer tracks • The sectors and tracks are mapped to a large 1-dimensional array of logical blocks, • Block 0 is the first sector of the first track on the outermost or innermost cylinder. • Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost. • The block is the smallest unit of transfer. Operating System Concepts

  4. Blocks and Fragments • Block sizes • Smaller => less internal fragmentation • Larger => larger transfers • Can improve efficiency by varying file-system parameters, e.g., block size = frame size on swap device • 4.2BSD uses two block sized for files which have no indirect blocks: • All the blocks of a file are of a large block size (such as 8K), except the last. • The last block is an appropriate multiple of a smaller fragment size (e.g., 1024) to fill out the file. • Thus, a file of size 18,000 bytes would have two 8K blocks and one 2K fragment (which would not be filled completely). Operating System Concepts

  5. High Level Format • To use a disk to hold files, the operating system still needs to record its own data structures on the disk. • Partition the disk into one or more groups of cylinders. • Logical formatting or “making a file system”. Operating System Concepts

  6. Partitions and Disks • A physical device may support several partitions, each hosting a physical file system. Operating System Concepts

  7. Partitions and Disks • Different file systems can support different uses. • Potentially different OSs in different partitions • Prevents one program from using all available space for a large file. • Speeds up searches on backup tapes and restoring partitions from tape. • Known as • IBM minidisks • Mac volumes • UNIX file systems • Windows drives Operating System Concepts

  8. Partitions • Boot block - for booting the OS • Partition control block • Information about the partition • Free space list • Free directory space list • Superblock in UNIX, Master File Table in NTFS • Directory structure • Directory nodes, exactly one per file • Inodes in UNIX • MFT entries in NTFS • Organization links nodes into a data structure • Provides naming • Data blocks Operating System Concepts

  9. Directory Implementation • Linear list of file names with pointer to the data blocks. • Simple to program • Inefficient - searching, empty entries • UNIX directory files • Sorted tree, e.g., B-tree • NTFS B+ tree • Hash Table – linear list with hash data structure. • Decreases directory search time • Collisions – situations where two file names hash to the same location • Fixed size, or overflow facilities Operating System Concepts

  10. Disk Structures • A logical file system that a user ordinarily sees may consist of several physical file systems, each hosted in a partition of a physical device. • Permits very large logical file systems • Makes monolithic backups possible • The root file system is always available on a partition. • Other file systems may be mounted — i.e., integrated into the directory hierarchy of the root file system. • A file system must be mounted before it can be accessed. • A unmounted file system mounted at a mount point. Operating System Concepts

  11. Mapping File System to Physical Devices Operating System Concepts

  12. Layered File System • Applications call the LFS via system calls • LFS deals with file names, file and directory operations, file tables, security • FOM maps logical to physical, buffers, tracks free blocks, tracks bad blocks • BFS sends commands to DDs to access specified blocks on the device • I/O control consists of device drivers and interrupts handlers for each device. Operating System Concepts

  13. File Tables • Draw my example • To open a file • System wide file table is search to see if already open • If no, the directory structure is searched (cached if possible) • Node copied to system wide file table • A new entry in the per-process file table • Refers to the existing SW file table entry • Contains mode of access, and offset • Counter incremented in the SW file table • To close a file • Per-process file table entry is removed • Count decremented in SW file table, and if 0 the node is written to disk • To create a file • LFS allocates a new directory node • Reads in, updates, and writes out the directory Operating System Concepts

  14. Contiguous Allocation • Each file occupies a set of contiguous blocks on the disk • E.g., IBM, VMS Operating System Concepts

  15. Contiguous Allocation • Random access Block = LA / BlockSize Offset = LA % BlockSize • Simple – only starting location (block #) and length (number of blocks) are required in a directory node • Wasteful of space • Dynamic storage-allocation problem • External fragmentation • Files cannot grow easily • Initial allocation? Operating System Concepts

  16. Linked Allocation pointer block = • Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. • Block is a pointer (disk address) and data Operating System Concepts

  17. Linked Allocation (Cont.) • Random access Block in chain = LA / (BlockSize - LinkSize) Offset = LA % (BlockSize - LinkSize) • Simple – need only starting address • Free-space management system – no waste of space • New file has NULL pointer in directory • Easy to get another or release a block • Slow random access Operating System Concepts

  18. Linked Allocation (Cont.) • FAT variation • Table with index corresponding to blocks • Linked table entries - cached for fast traversal • Used by DOS and OS/2 Operating System Concepts

  19. Extent-Based Systems • Many newer file systems (e.g., Veritas File System) use a modified contiguous allocation scheme. • Extent-based file systems allocate disk blocks in extents. • An extent is a contiguous block of disks. Extents are allocated for file allocation. A file consists of one or more extents. • Extents are linked together • Extents reduce LinkSize overhead • E.g., NTFS Operating System Concepts

  20. Indexed Allocation • Brings all pointers together into the index block. index table Operating System Concepts

  21. Indexed Allocation (Cont.) • Random access Index table entry = LA / BlockSize Offset = LA % BlockSize • Fast random access • Cached for speed • Need index block • Small files have mostly empty index block • Fixed maximal file size Operating System Concepts

  22. Linked - Indexed Allocation • Linked scheme – Link blocks of index table • Random access Index block in chain = LA / (((BlockSize-LinkSize)/LinkSize) * BlockSize) LAInBlock = LA % (((BlockSize-LinkSize)/LinkSize) * BlockSize) Index table entry = LAInBlock / BlockSize Offset = LAInBlock % BlockSize Operating System Concepts

  23. Two Level Indexed Allocation  outer-index file index table Operating System Concepts

  24. Two Level Indexed Allocation • Random access Outer index table entry = LA / (BlockSize2/LinkSize) LAInBlock = LA % (BlockSize2/LinkSize) Inner index table entry = LAInBlock / BlockSize Offset = LAInBlock % BlockSize • Fixed, but large maximal size • Fast random access if cached Operating System Concepts

  25. Combined Scheme: UNIX (4K bytes per block) • Fast for small files • Gradual degradation as file size increases Operating System Concepts

  26. Free-Space Management • Linked list (free list) • Cannot get contiguous space easily • No waste of space • Can use same routines as for file management Operating System Concepts

  27. Free-Space Management • Bit vector (n blocks) (e.g., Mac) 0 1 2 n-1 … 1  block[i] free 0  block[i] occupied  bit[i] = • Can use Motorola “find bit” instruction • Easy to get contiguous blocks • Needs to be kept in memory block size = 212 bytes (4KB) disk size = 236 bytes (64GB) n = 236/212 = 224 bits = 221 bytes (2MB (!)) Operating System Concepts

  28. Efficiency and Performance • Efficiency dependent on: • Disk allocation and directory algorithms • Types of data kept in file’s directory entry • Performance • Disk cache – separate section of main memory for frequently used blocks • Free-behind and read-ahead – techniques to optimize cache performance for sequential access • A page cache caches pages rather than disk blocks using virtual memory techniques. • Improve PC performance by dedicating section of memory as virtual disk, or RAM disk. Operating System Concepts

  29. Log Structured File Systems • Log structured (or journaling) file systems record each update to the file system as a transaction. • All transactions are written to a log. A transaction is considered committed once it is written to the log. However, the file system may not yet be updated. • The transactions in the log are asynchronously written to the file system. When the file system is modified, the transaction is removed from the log. • If the file system crashes, all remaining transactions in the log must still be performed. Operating System Concepts

  30. UNIX File Systems • Size of a block is given in sys/param.h as BSIZE, and is typically 512 or 1024 bytes. • The area of disc set aside for a file system is split into 4 regions: • Block 0 is set aside for a bootstrap program if required • Block 1 contains the superblock which describes the file system as a whole, including free blocks and inodes lists. • Block 2 starts the inode area (fixed size) • Remaining blocks are allocated to files • The UNIX file system supports two main objects: files and directories. • Directories are just files with a special format, so the representation of a file is the basic UNIX concept. Operating System Concepts

  31. Inodes • A file (could be a directory) is represented by an inode — a record that stores information about a specific file on the disk. • Type of file • Plain • Directory • FIFO • Special, … • Access permissions. • Number of links to this inode • User and group IDs of the file owner. • Size of file in bytes for files, device number for devices. • Times of creation, last access, and last modification • Addresses of the files data blocks. • Inodes are numbered from 1 • Inode 1 is reserved for the bad blocks list • Inode 2 for the root directory of the file system. Operating System Concepts

  32. Directories • The inode type field distinguishes between plain files and directories. • Directory entries are of variable length, with fields: • The length of the entry • The file name • The inode number. Operating System Concepts

  33. Finding Inodes • The OS has to map the supplied user path name to an inode • If the first character is “/”, the starting directory is the root directory. • For any other starting character, the starting directory is the current directory. • The search process continues until the end of the path name is reached and the desired inode is returned. • 4.3BSD improved file system performance by adding a directory name cache to hold recent directory-to-inode translations. Operating System Concepts

  34. File System Data Structures • UNIX maintains • System inode table - inodes of files that are open + name + counter • System file table - offset, mode + counter • Per process file table - not much • Tables are of fixed length Operating System Concepts

  35. Files • To create a file • Allocate new inode • Write new entry in directory file • To open a file • Search inode table • If open, create system & process file table entries, increment counters • If not open, read in inode and create table entries • To use a file, use the process file table entry • To close a file • Delete process file table entry, decrement system file table counter • If system file table counter is 0, delete and decrement inode table counter • If inode table counter is 0, write out inode and delete • To delete a file • Delete directory entry • Decrement link counter • If link counter is 0, and inode not in inode table, delete inode Operating System Concepts

  36. Schematic View of NFS Architecture Operating System Concepts

  37. NFS Protocol • Provides a set of remote procedure calls for remote file operations. The procedures support the following operations: • searching for a file within a directory • reading a set of directory entries • manipulating links and directories • accessing file attributes • reading and writing files • NFS servers are stateless; each request has to provide a full set of arguments. • Modified data must be committed to the server’s disk before results are returned to the client (lose advantages of caching). • The NFS protocol does not provide concurrency-control mechanisms. Operating System Concepts

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